The invention concerns a hydraulic steering arrangement with a steering handwheel which is connected with a manual pump, with a pump supplying hydraulic fluid, a steering motor and a steering valve arranged between pump and steering motor and having a valve element controlling the flow of hydraulic fluid between the pump and the steering motor, the valve element being displaceable by hydraulic energy originating from the manual pump.
Such a steering arrangement is known from for example DE 25 57 373 A1. The principle is not to use the amount of hydraulic fluid supplied during normal operation by the manual pump connected with the steering handwheel for direct operation of the steering motor, but only to displace a valve element, in this case a steering slide, of the steering valve. The steering slide then provides a connection between the pump, operated for example by the motor of the steered vehicle, and the steering motor, so that the steering motor is provided with hydraulic fluid originating from the pump. This is a reliable way of producing a steering with auxiliary power support.
In many cases, however, not only operation of the vehicle with the steering handwheel but also additional steering opportunities are wanted, for example from a second driver""s cabin or by means of a remote control. A revision of the xe2x80x9csteering ordersxe2x80x9d given by the steering handwheel is often also wanted, for example to improve the steering comfort of the vehicle. In vehicles with articulated steering, for example, it is advantageous to reduce the steering speed or steering acceleration to a certain value to avoid uncomfortable acceleration forces on the driver.
This can for example be realised in that, like in U.S. Pat. No. 4,955,445, a different sort of valve steering is used, namely an electrical steering. However, the disadvantage of this is that an additional error possibility occurs. Not only can the hydraulic circuit fail, but errors can also appear in the switching arrangement required for the displacement of the valve slide.
It is the task of the invention to provide a steering arrangement with additional functions without having to give up a hydraulic control.
In a hydraulic steering arrangement as mentioned in the introduction, this task is solved in that the valve element has an additional drive.
With this embodiment, the valve element can on one side be hydraulically controlled, when it is operated by the pressure of the manual pump. On the other side, however, it can also be displaced in a different way, namely by the additional drive mentioned. Also when the control opportunities for the additional drive fail, the normal function of the steering arrangement is maintained. The pressures produced by the manual pump on turning the steering handwheel can still act on the valve element. Thus, no additional steering valves are required to be connected parallel to the original steering valve to implement the wanted steering opportunities. On the contrary, a simple extension of the known steering valve by an additional drive for the valve element will cause an extension of the functions. The fact that no additional valves are required will save both space and costs. Further, such a steering arrangement will increase the steering comfort, as it is no longer necessary to balance several valves in relation to each other. The failure rate will be reduced accordingly.
In a preferred embodiment, the additional drive is electrically controllable. Many additional functions can be transmitted to the steering valve by electrical signals. The fact that the additional drive is electrically controllable makes it easy to process this information.
It is particularly preferred that the additional drive works electrohydraulically. The force provided by the additional drive is not, or not only, produced electrically. On the contrary, the electrical signals serve the purpose of controlling hydraulic pressures so that the wanted movement of the valve element occurs. However, these hydraulic pressures are available from the pump anyway. These pressures can for example be controlled via solenoid valves working in timed sequence, which enables the use of simpler solenoid valves. In practice, they must only be able to assume an open and a closed state. The pressures are then adjusted via the duty ratio.
Preferably, the pressures of the manual pump exert larger forces on the valve element than the additional drive. This means that the activation of the steering valve via the steering handwheel will always have preference over the activation through the additional drive, even though, if required, a movement of the valve element by means of the manual pump can override an additional movement. In practice this can be realised in that the activation pressure of the additional drive is kept relatively small.
Advantageously, the additional drive is connected with a control arrangement, which is connected with an external controller. This controller could for example be a control column or a joystick. With this control column the control arrangement can be told how the steering arrangement must work. The controller can also consist of a transmitting and a receiving unit, so that, if wanted, the steering arrangement can also be remote-controlled. The controller can also be a sensor sensing a trace in the track, so that the vehicle can also drive without a driver.
It is also advantageous to connect the steering arrangement with a sensor monitoring a variable size of the steering arrangement. Such a sensor can for example be used for the failure monitoring. Thus it can register, if the steering motor is displaced, also when a signal is not available, or if it is displaced sufficiently, when a signal is applied. By means of the sensor it is possible for example to deactivate the additional drive, if the travel gets to long. Also accelerations can be measured, in order to reduce uncomfortable loads on the driver, for example in connection with articulated vehicles.
Preferably, the sensor is a steering handwheel sensor. The steering handwheel sensor can for example monitor the rotation speed of the steering handwheel. To keep the transversal acceleration in connection with the steering of the vehicle within predetermined limits, the additional drive can then act against the pressures of the manual pump on the valve element, so that less hydraulic fluid than ordered by the driver by means of the steering handwheel reaches the steering motor from the pump. In fact, however, a random transfer function between the movement of the steering handwheel and the movement of the steering motor can also be produced. If wanted, this transfer function can also be changed in dependence of the speed.
In a preferred embodiment, the manual pump is arranged in a circuit with an orifice arrangement with at least one orifice, the orifice arrangement being connected with actuating connections of the steering valve and arranged parallel to the steering valve. The hydraulic fluid supplied by the pump thus flows in a closed circuit, when the steering handwheel is operated. In this circuit one or more orifices are arranged, which then produce a pressure drop between the two actuating connections of the steering valve. This pressure drop produces a pressure difference over the valve element, which is then used for actuating the valve element, that is for example for displacing a valve slide. This is advantageous in that the actuation of the valve element only takes place as long as the steering handwheel is operated. As soon as the steering handwheel is no longer turned, hydraulic fluid supply stops, and accordingly the orifice produces no pressure drop. The steering valve then returns to its neutral position, and the steering motor displaced during the operation of the steering handwheel remains in its displaced position. Thus the driver will have to deliberately turn back the steering handwheel to bring the wheels to the neutral position. On the other hand, however, the orifice also involves the advantage that a xe2x80x9cshort-circuit pathxe2x80x9d is available on operating the additional drive, through which path the hydraulic fluid displaced from one side of the valve element can flow back to the other side of the valve element, without risking that such a movement is blocked by the manual pump. This could for example be the case, when the steering handwheel is blocked and no fluid can flow through the manual pump.
Preferably, the orifice has an adjustable size. Thus the flow resistance and the pressure drop, which can be generated at the orifice, can be adjusted. The advantage is that the same steering arrangement can be used for different vehicles. The xe2x80x9ctransmission ratioxe2x80x9d between steering handwheel rotations and the point of impact of the steered vehicle wheels, or the articulation of the vehicle when articulated vehicles are concerned, can be adjusted by changing the orifice size. The orifice size can also be adjusted in dependence of the operating conditions. For example difficult ground often requires a different steering behaviour than normal road driving. If wanted, the adjustment of the orifice size can be speed dependent.
Preferably, a throttle is provided in the flow path between pump and steering motor, and the orifice is controlled so that the relation between the pressure drops over orifice and throttle follow a predetermined function. Thus, in principle, a flow amplifier can be realised. The relation then states the amplification factor with which the valve element is actuated in dependence of the movement of the steering handwheel. The relation can also be variable, so that further parameters, such as vehicle speed, ground properties, load etc., can influence the relation. It is also possible that the flow through the orifice is a predetermined function of the flow through the throttle.
In a particularly preferred embodiment, however, the relation is constant. Thus, pilot flow and steering flow must be proportional to each other, in order that a constant amplification factor can be obtained. Correspondingly, the valve element is only displaced by a certain distance depending on the steering handwheel speed.
Preferably, an additional valve arrangement is arranged between the manual pump and the steering valve, which connects the manual pump with either the steering valve or, when the pump pressure drops below a predetermined value, with the steering motor. This additional valve arrangement provides a safety measure, so that the steering arrangement is kept operational, also on pump failure resulting in a drop of the pump pressure. The steering motor is then operated direct via the steering handwheel. By way of the additional valve arrangement the steering valve is disconnected in this embodiment, so that here no additional resistances are produced, which could make the steering using the manual pump as emergency pump more difficult.
It is preferred that the additional valve arrangement has an additional valve element arrangement, whose steering inlet is connected with the outlet of the pump. Thus the steering inlet of the additional valve arrangement is always loaded with the pump pressure. The pump pressure can then always be used to position the additional valve element arrangement so that the manual pump is connected with the control inlets of the steering valve. In the opposite direction the additional valve element arrangement is then loaded by a counterforce, for example a spring. When now the pump pressure drops, the steering inlet no longer gets the required pressure, so that the additional valve element arrangement is displaced to its other position and the additional valve arrangement connects the manual pump direct with the steering motor.
Preferably, the additional valve arrangement has an additional valve on both sides of the manual pump. Of course, both additional valves can also be arranged in a housing. However, this gives a symmetric steering of the steering valve, meaning that in case of failure, that is a drop in the pump pressure, both sides of the manual pump are connected direct with the steering motor.